arXiv subjects

Susan N. Coppersmith

Publications and source records attributed to Susan N. Coppersmith.

18 recordsLinked to original sources

arXiv2026

Decoherence of Majorana qubits by 1/f noise

Qubits based on Majorana zero modes (MZMs) in superconductor-semiconductor nanowires have attracted intense interest due to claims that their error rates are suppressed exponentially with increasing nanowire length or decreasing temperature. However, here we show that these qubits are subject to substantial decoherence resulting from the high-frequency components of 1/f charge noise, which is ubiquitous in the materials surrounding the nanowire. This process excites quasiparticles in the bulk of the topological superconductor that cause qubit decoherence even under otherwise ideal conditions. Increasing nanowire capacitance suppresses this mechanism but exposes the qubits to decoherence from externally-generated quasiparticles. Therefore, achieving high-fidelity MZM qubits using superconductor-semiconductor nanowires will require engineering strategies and compromises very similar to those needed for conventional superconducting qubits.

cond-mat.mes-hall↗

arXiv2025

Stochastic Resonance Spectroscopy: Characterizing Fast Dynamics with Slow Measurements

A system's internal dynamics and its interaction with the environment can be determined by tracking how external perturbations affect its transition rates between states. Quantitative measurements of these rates are crucial for optimizing quantum systems at the atomic scale but are challenging, as these dynamics are often faster than experimental observation capabilities. Here, we show that driving a stochastic system periodically enables quantitative determination of its transition rates over a wide frequency range spanning from kilohertz to gigahertz. To perform this quantitative extraction, we provide an analytical model that is applicable irrespective of the details of the studied system. We name this method stochastic resonance spectroscopy (SRS). We apply it to quantify spin switching in individual magnetic atoms, Neel state reversal in nano-antiferromagnets and quasiparticle transport through Yu-Shiba-Rusinov states. We anticipate that the ability to characterize broadband stochastic dynamics at the atomic scale will enable insights into an even wider range of systems, e.g. dynamics driven by light-matter interaction. Another exciting prospect is the investigation of systems that exhibit non-Markovian dynamics due to correlations with the environment.

cond-mat.mes-hall↗

arXiv2025

Bayesian and geometric analyses of power spectral densities of spin qubits in Si/SiGe quantum dot devices

Charge noise with a power-law spectrum poses a significant challenge to high-fidelity operation of spin qubits in semiconductor devices. Recently, considerable experimental work characterized this noise using qubits as spectrometers. It apparently arises from a collection of two-level fluctuating electric dipole systems (TLS). This suggests using the data to infer the positions, orientations, and other physical characteristics of the TLS. We identify a fundamental difficulty in this program: the inference of the TLS parameters is strongly undetermined, since the quantity of data is not sufficient to fix them uniquely. We describe two approaches to deal with this situation. The first approach is a qualitative method based on analytic calculations and simulations of small model systems that recognizes certain patterns. The second approach, more appropriate for detailed data analysis, is a Bayesian computation that assigns probabilities to candidate dipole configurations. We propose that the Brier score, a measure of confidence in the probabilities, can be used as a quantitative tool to judge the efficacy of experimental noise-measurement setups. Together, the analytical and computational Bayesian methods constrain, but do not fix, the density, the positions, the orientations, and the strengths of the dipole noise sources.

quant-ph↗

arXiv2025

Uncertainties in tellurium-based dark matter searches stemming from nuclear structure uncertainties

Using tellurium dioxide as a target, we calculate uncertainties on 90% upper confidence limits of Galilean effective field theory (Galilean EFT) couplings to a weakly-interacting massive particle (WIMP) dark matter candidate due to uncertainties in nuclear shell models. We find that these uncertainties in naturally-occurring tellurium isotopes are comparable across the different Galilean EFT couplings to uncertainties in xenon, with some reaching over 100%. We also consider the effect these nuclear uncertainties have on estimates of the annual modulation of dark matter from these searches, finding that the uncertainties in the modulation amplitude are proportional to the non-modulating upper confidence limit uncertainties. We also show that the determination of the modulation phase is insensitive to changes in the nuclear model for a given isotope.

hep-ph↗

arXiv2024

Atomistic compositional details and their importance for spin qubits in isotope-purified silicon-germanium quantum wells

Understanding crystal characteristics down to the atomistic level increasingly emerges as a crucial insight for creating solid state platforms for qubits with reproducible and homogeneous properties. Here, isotope composition depth profiles in a SiGe/$^{28}$Si/SiGe heterostructure are analyzed with atom probe tomography (APT) and time-of-flight secondary-ion mass spectrometry. Spin-echo dephasing times $T_2^{echo}=128 μs$ and valley energy splittings around $200 μeV$ have been observed for single spin qubits in this quantum well (QW) heterostructure, pointing towards the suppression of qubit decoherence through hyperfine interaction or via scattering between valley states. The concentration of nuclear spin-carrying $^{29}$Si is 50 ppm in the $^{28}$Si QW. APT allows to uncover that both the top SiGe/$^{28}$Si and the bottom $^{28}$Si/SiGe interfaces of the QW are shaped by epitaxial growth front segregation signatures on a few monolayer scale. A subsequent thermal treatment broadens the top interface by about two monolayers, while the width of the bottom interface remains unchanged. Using a tight-binding model including SiGe alloy disorder, these experimental results suggest that the combination of the slightly thermally broadened top interface and of a minimal Ge concentration of $0.3 \%$ in the QW, resulting from segregation, is instrumental for the observed large valley splitting. Minimal Ge additions $< 1 \%$, which get more likely in thin QWs, will hence support high valley splitting without compromising coherence times. At the same time, taking thermal treatments during device processing as well as the occurrence of crystal growth characteristics into account seems important for the design of reproducible qubit properties.

cond-mat.mes-hall↗

arXiv2024

Uncertainties on the EFT coupling limits for direct dark matter detection experiments stemming from uncertainties of target properties

Direct detection experiments are still one of the most promising ways to unravel the nature of dark matter. To fully understand how well these experiments constrain the dark matter interactions with the Standard Model particles, all the uncertainties affecting the calculations must be known. It is especially critical now because direct detection experiments recently moved from placing limits only on the two elementary spin independent and spin dependent operators to the complete set of possible operators coupling dark matter and nuclei in nonrelativistic theory. In our work, we estimate the effect of nuclear configuration-interaction uncertainties on the exclusion bounds for one of the existing xenon-based experiments for all fifteen operators. We find that for operator number 13 the $\pm 1σ$ uncertainty on the coupling between the dark matter and nucleon can reach more than 50% for dark matter masses between 10 and 1000 GeV. In addition, we discuss how quantum computers can help to reduce this uncertainty and how the uncertainties are affected for couplings obtained for the nonrelativistic reductions of the relativistic interactions.

hep-ph↗

arXiv2024

Practical Strategies for Enhancing the Valley Splitting in Si/SiGe Quantum Wells

Silicon/silicon-germanium heterostructures have many important advantages for hosting spin qubits. However, controlling the valley splitting (the energy splitting between the two low-lying conduction-band valleys) remains a critical challenge for ensuring qubit reliability. Broad distributions of valley splittings are commonplace, even among quantum dots formed on the same chip. In this work, we theoretically explore the interplay between quantum-well imperfections that suppress the valley splitting and cause variability, such as broadened interfaces and atomic steps at the interface, while self-consistently accounting for germanium concentration fluctuations. We consider both conventional and unconventional approaches for controlling the valley splitting, and present concrete strategies for implementing them. Our results provide a clear path for achieving qubit uniformity in a scalable silicon quantum computer.

cond-mat.mes-hall↗

arXiv2023

Effects of Temperature Fluctuations on Charge Noise in Quantum Dot Qubits

Silicon quantum dot qubits show great promise but suffer from charge noise with a 1/f^αspectrum, where f is frequency and α\lesssim 1. It has recently been proposed that 1/f^αnoise spectra can emerge from a few thermally activated two-level fluctuators in the presence of sub-bath temperature fluctuations associated with a two-dimensional electron gas (2DEG)~\cite{Ahn2021}. We investigate this proposal by doing Monte Carlo simulations of a single Ising spin in a bath with a fluctuating temperature. We find that to obtain noise with a $1/f^α$ spectrum with $alpha \lesssim 1 down to low frequencies, the duration of temperature fluctuations must be comparable to the inverse of the lowest frequency at which the noise is measured. This result is consistent with an analytic calculation in which the fluctuator is a two-state system with dynamics governed by time-dependent switching rates. In this case we find that the noise spectrum follows a Lorentzian at frequencies lower than the inverse of the average duration of the lowest switching rate. We then estimate relaxation times of thermal fluctuations by considering thermal diffusion in an electron gas in a confined geometry. We conclude that temperature fluctuations in a 2DEG sub-bath would require an unphysically long duration to be consistent with experimental measurements of 1/f-like charge noise in quantum dots at frequencies extending well below 1 Hz.

cond-mat.mes-hall↗

arXiv2022

Atomic fluctuations lifting the energy degeneracy in Si/SiGe quantum dots

Electron spins in Si/SiGe quantum wells suffer from nearly degenerate conduction band valleys, which compete with the spin degree of freedom in the formation of qubits. Despite attempts to enhance the valley energy splitting deterministically, by engineering a sharp interface, valley splitting fluctuations remain a serious problem for qubit uniformity, needed to scale up to large quantum processors. Here, we elucidate and statistically predict the valley splitting by the holistic integration of 3D atomic-level properties, theory and transport. We find that the concentration fluctuations of Si and Ge atoms within the 3D landscape of Si/SiGe interfaces can explain the observed large spread of valley splitting from measurements on many quantum dot devices. Against the prevailing belief, we propose to boost these random alloy composition fluctuations by incorporating Ge atoms in the Si quantum well to statistically enhance valley splitting.

cond-mat.mes-hall↗

arXiv2021

Pauli Blockade in Silicon Quantum Dots with Spin-Orbit Control

Quantum computation relies on accurate measurements of qubits not only for reading the output of the calculation, but also to perform error correction. Most proposed scalable silicon architectures utilize Pauli blockade of triplet states for spin-to-charge conversion. In recent experiments, there have been instances when instead of conventional triplet blockade readout, Pauli blockade is sustained only between parallel spin configurations, with $|T_0\rangle$ relaxing quickly to the singlet state and leaving $|T_+\rangle$ and $|T_-\rangle$ states blockaded -- which we call \textit{parity readout}. Both types of blockade can be used for readout in quantum computing, but it is crucial to maximize the fidelity and understand in which regime the system operates. We devise and perform an experiment in which the crossover between parity and singlet-triplet readout can be identified by investigating the underlying physics of the $|T_0\rangle$ relaxation rate. This rate is tunable over four orders of magnitude by controlling the Zeeman energy difference between the dots induced by spin-orbit coupling, which in turn depends on the direction of the applied magnetic field. We suggest a theoretical model incorporating charge noise and relaxation effects that explains quantitatively our results. Investigating the model both analytically and numerically, we identify strategies to obtain on-demand either singlet-triplet or parity readout consistently across large arrays of dots. We also discuss how parity readout can be used to perform full two-qubit state tomography and its impact on quantum error detection schemes in large-scale silicon quantum computers.

quant-ph↗

arXiv2020

Effect of quantum Hall edge strips on valley splitting in silicon quantum wells

We determine the energy splitting of the conduction-band valleys in two-dimensional electrons confined to low-disorder Si quantum wells. We probe the valley splitting dependence on both perpendicular magnetic field $B$ and Hall density by performing activation energy measurements in the quantum Hall regime over a large range of filling factors. The mobility gap of the valley-split levels increases linearly with $B$ and is strikingly independent of Hall density. The data are consistent with a transport model in which valley splitting depends on the incremental changes in density $eB/h$ across quantum Hall edge strips, rather than the bulk density. Based on these results, we estimate that the valley splitting increases with density at a rate of 116 $μ$eV/10$^{11}$cm$^{-2}$, consistent with theoretical predictions for near-perfect quantum well top interfaces.

cond-mat.mes-hall↗

arXiv2020

Realization of a $CQ_3$ Qubit: energy spectroscopy and coherence

The energy landscape of a single electron in a triple quantum dot can be tuned such that the energy separation between ground and excited states becomes a flat function of the relevant gate voltages. These so-called sweet spots are beneficial for charge coherence, since the decoherence effects caused by small fluctuations of gate voltages or surrounding charge fluctuators are minimized. We propose a new operation point for a triple quantum dot charge qubit, a so-called $CQ_3$-qubit, having a third order sweet spot. We show strong coupling of the qubit to single photons in a frequency tunable high-impedance SQUID-array resonator. In the dispersive regime we investigate the qubit linewidth in the vicinity of the proposed operating point. In contrast to the expectation for a higher order sweet spot, we there find a local maximum of the linewidth. We find that this is due to a non-negligible contribution of noise on the quadrupolar detuning axis not being in a sweet spot at the proposed operating point. While the original motivation to realize a low-decoherence charge qubit was not fulfilled, our analysis provides insights into charge decoherence mechanisms relevant also for other qubits.

cond-mat.mes-hall↗

arXiv2019

Repetitive quantum non-demolition measurement and soft decoding of a silicon spin qubit

Quantum error correction is of crucial importance for fault-tolerant quantum computers. As an essential step towards the implementation of quantum error-correcting codes, quantum non-demolition (QND) measurements are needed to efficiently detect the state of a logical qubit without destroying it. Here we implement QND measurements in a Si/SiGe two-qubit system, with one qubit serving as the logical qubit and the other serving as the ancilla. Making use of a two-qubit controlled-rotation gate, the state of the logical qubit is mapped onto the ancilla, followed by a destructive readout of the ancilla. Repeating this procedure enhances the logical readout fidelity from $75.5\pm 0.3\%$ to $94.5 \pm 0.2\%$ after 15 ancilla readouts. In addition, we compare the conventional thresholding method with an improved signal processing method called soft decoding that makes use of analog information in the readout signal to better estimate the state of the logical qubit. We demonstrate that soft decoding leads to a significant reduction in the required number of repetitions when the readout errors become limited by Gaussian noise, for instance in the case of readouts with a low signal-to-noise ratio. These results pave the way for the implementation of quantum error correction with spin qubits in silicon.

quant-ph↗

arXiv2019

Spatial Noise Correlations in a Si/SiGe Two-Qubit Device from Bell State Coherences

We study spatial noise correlations in a Si/SiGe two-qubit device with integrated micromagnets. Our method relies on the concept of decoherence-free subspaces, whereby we measure the coherence time for two different Bell states, designed to be sensitive only to either correlated or anti-correlated noise respectively. From these measurements, we find weak correlations in low-frequency noise acting on the two qubits, while no correlations could be detected in high-frequency noise. A theoretical model and numerical simulations give further insight into the additive effect of multiple independent (anti-)correlated noise sources with an asymmetric effect on the two qubits. Such a scenario is plausible given the data and our understanding of the physics of this system. This work is highly relevant for the design of optimized quantum error correction codes for spin qubits in quantum dot arrays, as well as for optimizing the design of future quantum dot arrays.

cond-mat.mes-hall↗

arXiv2019

Strong photon coupling to the quadrupole moment of an electron in solid state

The implementation of circuit quantum electrodynamics allows coupling distant qubits by microwave photons hosted in on-chip superconducting resonators. Typically, the qubit-photon interaction is realized by coupling the photons to the electric dipole moment of the qubit. A recent proposal suggests storing the quantum information in the electric quadrupole moment of an electron in a triple quantum dot. The qubit is expected to have improved coherence since it is insensitive to dipolar noise produced by distant voltage fluctuators. Here we experimentally realize a quadrupole qubit in a linear array of three quantum dots in a GaAs/AlGaAs heterostructure. A high impedance microwave resonator coupled to the middle dot interacts with the qubit quadrupole moment. We demonstrate strong quadrupole qubit--photon coupling and observe improved coherence properties when operating the qubit in the parameter space where the dipole coupling vanishes.

cond-mat.mes-hall↗

arXiv2013

Silicon Quantum Electronics

This review describes recent groundbreaking results in Si, Si/SiGe and dopant-based quantum dots, and it highlights the remarkable advances in Si-based quantum physics that have occurred in the past few years. This progress has been possible thanks to materials development for both Si quantum devices, and thanks to the physical understanding of quantum effects in silicon. Recent critical steps include the isolation of single electrons, the observation of spin blockade and single-shot read-out of individual electron spins in both dopants and gated quantum dots in Si. Each of these results has come with physics that was not anticipated from previous work in other material systems. These advances underline the significant progress towards the realization of spin quantum bits in a material with a long spin coherence time, crucial for quantum computation and spintronics.

cond-mat.mes-hall↗

arXiv2007

Ordering in red abalone nacre

Red abalone nacre is an intensely studied biomineral, and yet its formation mechanism remains poorly understood. Here we report quantitative measurements probing the degree of order of the aragonite tablets in nacre, and show that order develops over a distance of about 50 microns. These observations indicate that the orientational order of aragonite tablets in nacre is established gradually and dynamically, and we show that a model of controlled assembly based on suppression of the crystal growth rate along a specific direction, when growth is confined in a layered structure, yields a tablet pattern consistent with those revealed by detailed experimental measurements. This work provides strong evidence that the organism s control of crystal orientation in nacre occurs via regulation of crystal nucleation and growth as opposed to direct templation or heteroepitaxial growth on organic molecules on the organic matrix sheets.

cond-mat.mtrl-sci↗

arXiv1998

Determining Pair Interactions from Structural Correlations

We examine metastable configurations of a two-dimensional system of interacting particles on a quenched random potential landscape and ask how the configurational pair correlation function is related to the particle interactions and the statistical properties of the potential landscape. Understanding this relation facilitates quantitative studies of magnetic flux line interactions in type II superconductors, using structural information available from Lorentz microscope images or Bitter decorations. Previous work by some of us supported the conjecture that the relationship between pair correlations and interactions in pinned flux line ensembles is analogous to the corresponding relationship in the theory of simple liquids. The present paper aims at a more thorough understanding of this relation. We report the results of numerical simulations and present a theory for the low density behavior of the pair correlation function which agrees well with our simulations and captures features observed in experiments. In particular, we find that the resulting description goes beyond the conjectured classical liquid type relation and we remark on the differences.

cond-mat.supr-con↗